Looks like the Creative Zen Micro is the nearest serious rival to the iPod so far - this quote from the MacMinute post threadlinked above:

In a research note to clients Friday, Piper Jaffray analyst Gene Munster said the iPod still has no equal, despite a number of new entries coming from CES this week. "After the introduction of a handful of new portable audio devices yesterday at CES, we believe that the iPod is still in position to dominate this market," Munster said. "Of the other players looking to chip away at Apple's market share, Creative is gaining the most traction with its Creative Zen Micro. We believe that Creative may have had the most crowded booth at CES.

~ Piper Jaffray